Europa, with 30 square kilometers (km2) of land area, is 6-7 km in diameter with a maximum altitude of 6 meters (m). Its shallow lagoon (generally 1 m deep) is open to the sea at one side, and covers about 900 hectares (ha), including some 700 ha of mangrove swamp that is particularly extensive to the south. The lagoon is almost entirely exposed at low tide, and supports sparse seagrass beds at its outflow. These are dominated by Thalassodendron and Halodule species. Another smaller lagoon supports a mixed vegetation. The rim of the atoll is a karst structure; there is also a quantity of exposed coral rock and extensive sand dunes and rubble to the southwest. Europa is estimated to have formed about 90,000 years ago. It is surrounded by an abraded coral platform 200 to 600 m in width, which emerges at low tide carpeted with algae.

These islands are subject to the Agulhas Current and experience occasional cyclones. Water temperatures are usually above 30°C, and southeast trade winds dominate during the austral winter. Annual rainfall averages at about 600 millimeters (mm).

Ecology

Europa Island, despite its small area, presents a diverse flora and fauna, including several native vegetation types, a large seabird population, and a few endemic subspecies of birds and reptiles. Europa also supports one of the world's last stable populations of nesting green turtle (Chelonia mydas) that has not been seriously exploited. Unfortunately, Europa has shared a similar fate with many of the world's isolated islands, having suffered significant environmental damage from direct human alteration as well as from degradation by introduced species. This is most apparent when considering the large population of now feral goats inhabiting Europa.

Four vegetation types are identified on the island of Europa. These include a dry forest of silver thicket (Euphorbia stenoclada), a dry herbaceous formation dominated by a grass, Sclerodactylon macrostachyum, mangrove swamps of Rhizophora mucronata, and a coastal shrub formation that includes bay cedar (Suriana maritima). An abandoned plantation of introduced sisal (Agave sisalana) growing with Mauritius hemp (Furcraea foetida) is found on the northern end of the island. Sisal was planted by colonists who attempted to settle the island at the beginning of the 20th century. While the human settlement failed, the sisal persisted. It appears that the sisal is spreading; it now occurs in small islands within the Euphorbia thicket. The presence of this plantation cuts down the region of Euphorbia stenoclada by about 10%. About fifteen other plant species were introduced intentionally or accidentally on Europa Island; four species have an unspecified origin but are most likely introduced. There are no endemic plant species on the island.

Biodiversity Features

Europa Island functions as an important nesting ground for sea turtles and seabirds, and also claims an endemic subspecies of land bird as well as two endemic subspecies of lizard. While Bassas da India is of scientific and biogeographical interest due to its relatively undisturbed reef system, the limited area of permanently dry land is not sufficient to support significant terrestrial life. Europa Island is considered one of the most important nesting sites for green turtle (Chelonia mydas) in the world. Chelonia mydas, the largest of the hard-shelled sea turtles, was classified as an endangered species on the IUCN Red List in 1996. Between 8,000 and 15,000 females nest on the island's beaches each year, and 0.7 to 2.4 million juvenile turtles hatch annually. Only about a dozen large populations of nesting females are known to persist worldwide, and only Europa and Raine Island (Australia) appear to have stable populations that are not heavily exploited. In a good year, over 10,000 females may nest on Europa. The hawksbill turtle (Eretmochelys imbricata) is also fairly common on the island.

A few of Europa's seabird populations appear to be genetically isolated from other populations of the Indian Ocean. White-tailed tropicbird (Phaethon lepturus) occurs as a distinctive morph on Europa Island. Its size is smaller than those of other populations within the Indian Ocean, more closely resembling the subspecies P. l. ascensionis from the central and eastern Atlantic Ocean and P. l. dorotheae from the Pacific Ocean. The Europa Island population also has a high frequency of golden morphs, a feature that is not present elsewhere in the western Indian Ocean. These differences lead scientists to believe that the Europa population of white-tailed tropicbird is isolated from other nearby colonies in the Indian Ocean; it has been proposed as a distinct subspecies, Phaethon lepturus europae, by Le Corre and Jouventin.

The pantropical red-footed booby (Sula sula) is another example of an isolated population. One of the most polymorphic seabirds, most extant colonies of the Indian Ocean have white morph adults. On Europa, however, adults are almost entirely of a white-tailed brown morph, differing significantly even from nearby colonies. Le Corre suggests that the occurrence of the white-tailed brown morph at Europa may act as a defensive camouflage against kleptoparasitism (a behavior of stealing another bird's catch) by great frigatebirds (Fregata minor) and brown skuas (Catharacta antarctica). A third example of the geographic isolation of Europa's seabirds is seen in the island's population of Audubon's Shearwater (Puffinus lherminieri), recently evidenced to belong to the subspecies P. l. bailloni. This subspecies was previously thought to be endemic to the Mascarenes Islands off the eastern shore of Madagascar. P. l. bailloni is distinct from the three subspecies of the Comoro, Aldabra, and Seychelles group to the north of Madagascar, suggesting that little exchange occurs north along the corridor of the Mozambique Channel.

Europa supports at least five species of land birds, including the endemic subspecies of Madagascar white-eye (Zosterops maderaspatana voeltzkowi'). This bird has managed to colonize all islands of the western Indian Ocean and to then diversify into numerous endemic subspecies or species. Other land birds include the dimorphic egret (Egretta dimorpha) (native to Europa, also present on Madagascar), Madagascar pond heron (Ardeola idea) (native to Europa), barn owl (Tyto alba) (possibly indigenous, though dependent on the introducedRattus rattus), and pied crow (Corvus albus) (probably introduced and dependent on the presence of humans).

Other species of interest on Europa Island are two endemic subspecies of lizard, Mabuya comorensis infralineata and Cryptoblepharus boutonii bitaeniatus. Over 55 species of insect make their home on these islands, including an endemic Chalcidoid wasp species that is a specialized parasite of the red wood ant.

Current Status

The fragile ecosystems of these small islands (Europa and Bassa da india) face threats common to most isolated islands. In addition to the physical alteration of the Euphorbia vegetation by humans, introduced species of plants and animals have had a significant effect on the island's ecology. Sisal is an invasive species in many localities globally, having the ability to out-compete native vegetation. Goats (Capra hircus) were introduced from Madagascar in 1860, by 1976 there were approximately 200, and their numbers have grown since. Particularly well-adapted to difficult environmental conditions, such as a lack of fresh water, goats are capable of dramatically altering vegetation, thereby destroying habitat vital to the island's native bird population. Their presence is particularly damaging to the native Euphorbe arborescente thickets as well as to the rare portia tree (Thespesia populnea) that grows in association with mangroves. Black rats (Rattus rattus) have been on the island since the 16th century and are efficient predators of the eggs and young of many seabirds. Donkeys, cats, and rabbits were also introduced to the island, but are now extinct.

A meteorological station was built on Europa Island in 1950, along with an airstrip. The station has been intermittently manned by French meteorologists or military personnel, with researchers visiting periodically. This human presence affects the native xeric scrub habitat, though the extent of the damage is unclear. Installation of the military airstrip undoubtedly had some effect, as does the continued presence of people on this small, fragile island. Both Europa and Bassas da India are classified as Nature Reserves, a type of low-level protection by order of the Prefect in 1975 and 1981. Fishing in territorial waters around the islands is banned, and anyone visiting the islands is expected to observe certain regulations. This type of reserve classification is not thought to provide maximum environmental protection, and proposals have been made for more effective protection measures.

Types and Severity of Threats

Introduced species both present and possible non native species continue to threaten this ecoregion. The lack of formal protection for the islands and continued visitation by humans increase the risk of introductions as well as problems associated with the use of resources. Furthermore, the Mozambique Channel is becoming polluted from heavy vessel traffic, and egg collection on these remote islands is also problematic.